First up, Microwave Ovens are very dangerous items to work on so unless you are qualified to do so DO NOT attempt repairs yourself. Call a repair man. NOTE. Some repairs are just not cost effective so it may be cheaper just to replace the microwave oven.FYI. The most likely issue will be one of the following

Magnetron is fried. Resistance between the magnetron terminals should be less than 3 ohms. Resistance from the magnetron terminal to ground should be infinite.

Fried HV capacitor. Use your ohm meter to test it.

Fried control board. If power is going to the board but not going off to the other components, the board is bad and needs to either be replaced or track down the bad connection (GE / Hotpoint).

HV Rectifier is shorted or open. Test forward and reverse bias with a megohmmeter. If continuity in both directions, rectifier is shorted, replace. If no continuity in either direction, rectifier is open, replace.

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If the level of "hum" from it is the same or less tha before, then you might have an open high voltage rectifier, open high voltage capacitor, or defective magnetron. If the noise level is higher, then one of these could be shorted.

Fully discharge the capacitor before testing to avoid shock, or worse.

You really don't want to take them apart do to radiation. Take the screws out in back plate. Carefully and remove and look in to see if you see black burnt spots. If you see any then chances are the main board shorted out on microwave.DO NOT STICK ANY TOOL IN MICROWAVE IN BACK. Some do have a round fuse in back.If your does take it out with some heavy plastic. Make sure before you take back off unplug from wall and the push start 3 times with 5 second apart. This will allow all electric in the power reducer to discharge. then you can work on it. On sides of use it will tell you what amps it is. Good Luck

Dear Kaiyi,
What has most likely happened is that the "magnetron" has failed.
The magnetron is a large metal object that emits sound waves that cook the food.
Two more options are the fuse inside, ( if there is one) or the capacitor that starts the magnetron.
By all means, someone will need tho tear it all apart and diagnose it. Many town and cities have no shop to do this, because the cost of the labor to go through all this is higher than the cost of a new one. God bless your efforts.

If there's a loud noise when you try to microwave food, then the current through the high voltage transformer is probably too high.
Most likely, this is being caused by failure of a component in the high voltage circuit. The most likely component is the high voltage rectifier, followed by the magnetron or high voltage capacitor. High voltage transformers occasionally do burn up as well.

I understand that you are noticing less noise, after about a second of starting your Frigidaire microwave, model# PLMB186CC. I suggest checking to see if the power level was changed. The blower sound will be different at levels other than "HIGH". As long as it is still heating, there is no cause for alarm. I ask that you please follow up with a comment on the post, at your convenience, to advise if further troubleshooting is needed or if your washer status has changed successfully. Hope this is helpful.

It sounds like the magnetron may have
either failed or is not being supplied with the extremely high
voltage required to run it.Make sure the !!!!capacitor is
discharged!!!!! before attempting any sort of repair.Check
the door interlock switches first then the high voltage diode with
either an AVO model 8 on high resistance range for short circuit, the
capacitor can fail and go short circuit, the feed fuse on the primary
of the high voltage transformer and then finally, the magnetron is
best checked by substitution.Hope that helps.

if its making a loud humming noise you need to remove the outer and carefully discharge the high-voltage capacitor in the magnetron circuitry(caps are usually oval in shape and 3 to 5 inches long) after this disconnect one terminal of the cap and check it for being open,then do the same on the high-voltage diode and check this for front to back resistance high in one direction and lower with the meters leads reversed on the diode,the humming is coming from the transformer not being loaded by the cap and diode to double the voltage required by the magnetron tube of -2,000 volts D.C.

Is the cooking level set to high? most microwaves you can adjust the cooking level. 10 is usually the highest. When you turn it on, there is usually an icon or indicator light that tells you you are cooking on high somewhere on the display

The noise is coming from the high-voltage transformer being overloaded
by something shorting it out. Either the magnatron or the
high-voltage capacitor and diode. Replace the capacitor and
diode then if still not working, replace the magnatron. Its only
affordable if you do it yourself - buy the part on EBAY - lookup
"universal magnatron"
Hope ya fix it!